1. Field of the Invention
This invention relates to musical tone synthesis and in particular is concerned with an improvement for producing tones from stored musical waveshapes.
2. Description of the Prior Art
A wide variety of musical tone generation systems have been designed which attempt to realistically replicate the sounds produced by conventional acoustic musical instruments. In general these systems have produced only poor imitative sounds because they lack the capability to produce the complex time variations of the musical waveform that characterizes a tone from a particular acoustic musical instrument.
The most obvious method to imitate a musical instrument is to record the sound and to replay these recordings in response to an actuated keyswitch. While at first thought the straightforward technique or recording and keyed playback seems to be attractive, a practical realization of such a musical instrument can be burdened by a large amount of memory required to store the recorded data. The maximum amount of storage is associated with a system that uses a separate and distinct recording for each note played in the range of the musical instrument's keyboard. Some economy in the storage requirement has been made by using a single recording for several contiguous musical notes. This assumes that the waveshape for the imitated musical instrument does not change greatly between several successive notes.
Electronic musical tone generators that operate by playing back recorded musical waveshapes stored in a binary digital data format have been given the generic name of PCM (Pulse Code Modulation). This is an unfortunate label because PCM can mean almost anything. In particular PCM in no way simply identifies the tone generator as one in which a recorded tone is simply stored in a binary digital data format. A musical instrument of the PCM type is described in U.S. Pat. No. 4,383,462 entitled "Electronic Musical Instrument." In the system described in the patent, the complete waveshape of a musical tone is stored for the attack and decay portions of the musical tone. A second memory is used to store the remainder of the tone which comprises the release phase of the musical tone. The sustain phase of the musical tone is obtained by using a third memory which stores only points for a single period of a waveshape. After the end of the decay phase, the data stored in the third memory is read out repetitively and the output data is multiplied by an envelope function generator to create the amplitude variation for the sustain and release portions of the generated musical tone.
A method of reducing the amount of stored data in a simple PCM musical tone generator is described in U.S. Pat. No. 3,763,364 entitled "Apparatus For Storing And Reading Out Periodic Waveforms." The disclosed system utilizes the even and/or odd symmetry of a waveform in such a manner that only sample points for one-half of the waveform need be stored in a waveshape memory. When the waveform is read from the waveshape memory, the unstored sample points of the second half of the waveform are recreated by using the symmetry property of the stored data points.